Simplify State Explosion in XState through Hierarchical States

Kyle Shevlin
InstructorKyle Shevlin

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Published 3 years ago
Updated 2 years ago

As our state machines grow more complex, we might find ourselves in a situation where a state should only exist if the machine is already in another state. To do this, we can use hierarchical states.

Instead of attempting to define all the states of a machine at a top level, we can nest states that should only be available as children of a parent state. These substates are written exactly like the states of the top level of a machine: with an initial property, and the states of the nested state graph.

Using hierarchical states is a great way to avoid needing to check for booleans in an application. If a state can only exist when another state exists, consider if the one isn't in fact a child of the other.

Instructor: [00:00] Here I have a state machine of a door. It's in the initial state of locked. I've written out the states locked, unlocked, closed, and open, but I haven't given them any events or transitions yet. That's because when we think about a door, it's actually a bit of challenging problem.

[00:16] When it's locked, it's also closed. When it's unlocked, it could still be closed, but it could also be opened. When it's opened, it should never be able to be locked. We end up with a pretty confusing graph of states if we try really hard to make this work with all the states on the same level.

[00:33] Fortunately for us, we don't have to keep them at the same level and we can use hierarchical states. We can see that closed and open really are states that fall under when the door is unlocked, and it might make more sense for us to put closed and open as states under unlocked. This is our first step in making hierarchical states.

[00:53] The next thing is we actually need to define what's the initial state of this subset of states. We can update our machine, and it now reflects that we have a locked state and an unlocked state. Inside the unlocked state, we have child states closed and open.

[01:09] From here, we can start to fill out our events and they'll make sense. When we're closed, we can open it. When we're opened, we can close it. We'll update that. When we're locked, we can get to unlocked. How do we go from our unlocked state back to our locked state?

[01:27] We don't want to be able to lock the door when it's open, so we don't want to put that event here. We can put an event when it's closed to lock it. However, this is going to throw an error when we update. It's going to say invalid transition for door unlocked closed.

[01:42] What it's saying is that locked doesn't exist as a state in unlocked. It's one level up. So how do we do this? We could start by using the ID on the machine itself, then doing dot notation to work our way down.

[01:57] In this case, we can set the transition target to door locked, and it'll go from door down to the locked state. We can see we have a lock action. Our door actually works. We've unlocked it, we can open it. We can't lock it from the open state. Closed, locked, and we're back to locked.

[02:15] The other way we can handle this, rather than using the identifier of the door, we can give the lock state an ID, and then we can change this just to locked. We update it, and our visualization stays the same. We can unlock the door, open, close, lock it.